Allow MR pipelines in latest DAST template

What does this MR do?

Related to #217668 (closed)

As per my proposal in #217668 (comment 502308480), this updates the DAST template to also work with Merge Request pipelines, reusing the technique shown in https://docs.gitlab.com/ee/ci/yaml/README.html#switch-between-branch-pipelines-and-merge-request-pipelines

• If users have workflow: rules that prevent merge request pipelines, there will be no change.
• If users have workflow: rules that prevent branch pipelines, and only MR pipelines, they will finally be able to use this template without overriding the job rules to match their pipeline. It can be added as-is to new pipelines, and those who already override with their own rules won't be affected.
• If users have config that allows both branch and MR pipelines, the job will run in branch pipelines for every push to the branch, until an MR is created. Then it will stop running in branch pipelines, and start running in MR pipelines, without duplication, thanks to the use of the $CI_OPEN_MERGE_REQUESTS variable.

Another way to look at it is that it "prefers" MR pipelines, but if there is no MR, or MR pipelines are disabled, it'll run in branch pipelines.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
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Security

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